Traffic Message Channel (TMC) is a technology for delivering traffic and travel information to motor vehicle drivers. It is digitally coded using the ALERT C or TPEG protocol into Radio Data System (RDS)Dietmar Kopitz, Bev Marks. RDS: The Radio Data System. Artech House, 1999. carried via conventional FM radio broadcasts. It can also be transmitted on Digital Audio Broadcasting or satellite radio. TMC allows silent delivery of dynamic information suitable for reproduction or display in the user's language without interrupting audio broadcast services. Both public and commercial services are operational in many countries. When data is integrated directly into a navigation system, traffic information can be used in the system's route calculation.
Development Detailed technical proposals for an RDS-TMC broadcasting protocol were first developed in the European Community's DRIVE programmeProposal for a Council Regulation (EEC) on a Community programme in the field of information technology and telecommunications applied to road transport – Drive (Dedicated road infrastructure for vehicle safety in Europe). COM(87) 351 final. research project RDS-ALERT, a partnership of the BBC, Philips, Blaupunkt, TRRL and CCETT led by Castle Rock Consultants (CRC).Castle Rock Consultants. Radio Data System (RDS) Traffic Message Channel (TMC). Final Report to the Commission of the European Communities, DRIVE Project V1029, Nottingham, UK, October 1988. The main goal of the project was to develop and build consensus upon a draft standard for broadcasting RDS-TMC traffic messages in densely coded digital form.S.R. Ely, BBC Research Department, Engineering Division, Kingswood Warren, Tadworth, UK. RDS-ALERT: a DRIVE project to develop a proposed standard for the Traffic Message Channel feature of the radio data system RDS, IEE Colloquium on 'The Car and its Environment – What DRIVE and PROMETHEUS Have to Offer (Digest No.20), 1990.
An initial proposal for defining RDS-TMC data fields had been made to the European Conference of Ministers of Transport (ECMT) in Madrid, based on a scheme developed by CCETT and Philips in the Eureka-sponsored CARMINAT research project.CARMINAT, Programme EUREKA (EU 55) : Realisation d'un système complet d'information, de géstion et de navigation à vocation Européenne pour les véhicules routiers [CARMINAT, EUREKA Programme (EU 55) : Development of a complete European information management and navigation system for road vehicles.] International Road and Traffic Conference. Roads and Traffic 2000. Vol. 1., p. 147-150. Text in French with summary in English. Coordinated by: Heusch/Boesefeldt G.m.b.H., Aachen Germany. Publication Date: 1988. This proposal required the use of at least two 104-bit RDS data groups for each message. Within these RDS Groups, 32 bits per group would be used for traffic data, giving a total traffic message length of 64 bits. A second proposal, by Bosch-Blaupunkt and the German Road Research Institute BASt, sought to use just a single RDS Group per traffic message. Then, in 1987, the CEC invited Castle Rock Consultants to lead a joint team that would take TMC development a stage further.J.L. Riley and A.K. McParland. RDS-ALERT: A proposed Traffic Message Channel (TMC) feature for the Radio Data System (RDS). BBC RD 1991/16, Research Department, Engineering Division, The British Broadcasting Corporation. CRC produced a proposal for a modified BASt/Blaupunkt single group message definition, which became known as the ALERT A coding scheme. Tests also continued at CCETT and BBC on the CARMINAT approach, which formed the basis of an alternative ALERT B coding proposal.
A major question addressed in the Alert A scheme was the total number of traffic event locations to be coded. Initial estimates suggested that, in Europe, a maximum of 65,000 significant junctions might be needed for the Federal Republic of Germany. An efficient coding system would require only 16 bits to code these, simply by numbering each intersection from 1 to 65535. Calculations for France, Britain and elsewhere suggested that around 30,000 to 40,000 locations should be enough for most European national or U.S. statewide systems. A standard 16-bit location code was, therefore, adopted for inter-urban networks. The Madrid proposal of 1987, by comparison, had required 33 bits to code problem location, with separate fields for road number, road class, area of the country, etc. These 33 bits gave a theoretical total of 8.5 billion location codes, most of which could never be used.Peter Davies, Chris Hill, and Grant Klein. Standards for the Radio Data System – Traffic Message Channel. Castle Rock Consultants, Leesburg, Virginia, USA. Vehicles/Highway Automation: Technology and Policy Issues (SP·791). Future Transportation Technology Conference and Exposition, Vancouver, BC, Canada. August 7·10, 1989
After consultation with ECMT, a combined approach was developed called the ALERT C Protocol that aimed to combine the best features of each approach. ALERT A and C replaced the CARMINAT message categories cause, effect and advice with a single 11-bit basic message code. This permits up to 2048 basic message phrases to be broadcast. The new ALERT protocols significantly increased the efficiency of message coding, shortening the basic message content from 18 to 11 bits. In conjunction with the revised location codes, which saved 17 of the 33 bits previously assigned, this allowed the great majority of traffic messages to be broadcast using a single TMC data sequence. In 1991, ECMT recommended moving forward with further testing of the protocols.European Conference of Ministers of Transport. Resolution No. 91/4 on Standards for Traffic Messages Broadcast using the Radio Data System Traffic Message Channel. [CEMT/CM(91)12] The work continued with a larger consortium including Volvo and Ford Motor Company in the European Commission's DRIVE II project ATT-ALERT.Davies, P.; Milton, H. Castle Rock Consultants, Nottingham, UK. The development of integrated traveller information protocols.Proceedings of the Intelligent Vehicles '94 Symposium, IEEE, 24-26 Oct. 1994, pages 602-605.
Operation Each traffic incident is binary-encoded and sent as a TMC message. Each message consists of an event code, location code, expected incident duration, affected extent and other details.
The message contains a list of up to 2048 event phrases defined by 11 binary bits (of which 1402 were in use as of 2007) that can be translated by the receiver into the user's language. Some phrases describe individual situations such as a crash, while others cover combinations of events such as construction causing long delays.
In Europe, location code tables are maintained on a national level. Those location tables are integrated in the maps provided by in-vehicle navigation system companies such as HERE Technologies and TomTom and by vehicle manufacturers such as Volvo. In other countries, such as the U.S. and Canada, private companies maintain the location tables and market TMC services commercially.
Sources of traffic information typically include police, traffic control centers, camera systems, traffic speed detectors, floating car data, winter driving reports and roadwork reports.
Coordination TMC-Forum, a non-profit organization whose members included service providers, receiver manufacturers, car manufacturers, map vendors, broadcasters (public and private), automobile clubs, and public authorities, was a forum to discuss traffic information related matters. It maintained the TMC-Standard (ISO 14819). On 11 November 2007, the TMC-Forum and the TPEG-Forum merged into the Traveller Information Services Association (TISA). TISA has taken over all of TMC-Forum's activities and responsibilities.
Functionality RDS-TMC is a low-bandwidth system. Each RDS-TMC traffic message comprises 37 data bits sent at most 1–3 times per second, using a low capacity data channel primarily designed for FM radio station name identification and tuning. Compressing traffic incident descriptions in multiple languages into 16 bits for a location, 11 bits for an event description code, plus 3 bits for the event's extent and a few extra bits for the duration/system management was necessary due to pre-existing constraints in the RDS standard. Almost all the other broadcast data bits were already assigned from each 104-bit RDS Group.
A major design challenge of RDS-TMC was to find a way of describing traffic event locations across an entire state or country. Such a system could not convey precise latitude-longitude data (available 25 years later using GPS in applications such as Waze). Instead, RDS-TMC relies on the use of location tables that point only to significant highway junctions. The precision of each traffic event's location is low compared to that of modern smartphone devices. The user's navigation system locates a driver to about 3 metres (10 feet), but only knows, for example, that a crash took place between Exit 3 and Exit 4, northbound on a particular motorway. This limitation requires that traffic events (accidents, congestion, burst water mains, faulty traffic lights, etc.) have to be superimposed onto maps by mapping the reported location to the TMC location table. If the nearest location table point lies at some distance from the exact position of the incident, then the report appears on a section of main road between two junctions instead of at its exact location. The limited precision can make a significant difference as to how navigation devices interpret the incident, potentially leading to an occasional poor route choice.
In the US and elsewhere, systems such as CARS (Condition Acquisition and Reporting System)P. Davies, K. Virshbo, Castle Rock Associates, Portland, Oregon, USA. CARS (Condition Acquisition Reporting System) and the CARS Group. Archives of Transport System Telematics, Volume 2, Issue 1, February 2009.Dennis W. Johnson.Congress Online: Bridging the Gap Between Citizens and Their Representatives p41. can pinpoint event locations or their start and end points with one-metre precision. These real-time data are published in XML for access by companies such as Google and TomTom. These incident reports can be delivered to mobile phones and handheld devices in vehicles. However, major real-world traffic incidents usually spread from hundreds of metres up to many kilometres, once traffic backups have developed. On motorways and other major roads, there are typically few or no detours available between significant junctions, which are all included in the TMC location tables. Many traffic report locations are only approximate, and as queues grow, locations can change swiftly. So GPS-based systems are more precise, but are not necessarily more accurate.
Security In April 2007, two Italian security researchers presented research about RDS-TMC at the CanSecWest security conference. The presentation, entitled "Unusual Car Navigation Tricks", raised the point that RDS-TMC is a wireless cleartext protocol and showed how to build a receiver and transmitter with inexpensive electronics capable of injecting false and potentially dangerous messages.
Detailed instructions and schematics were published in Issue No. 64 of Phrack hacking magazine.
The TMC Forum responded by stating that the effects of any 'pirate' TMC broadcasts would be non-existent on users not on routes affected by fake obstruction messages and that such broadcasts would directly interfere with that country's TMC carrier station, which would lead to criminal or civil liability. They stated that it was therefore unlikely that such activity would take place.
Actual RDS-TMC attacks have been known to occur, for instance in Belgium in 2019 where road users were warned of "air raids on the E40 road" in March and that "firefights broke out on the E17" in August